The present invention relates to an electric pole for a low-voltage power circuit breaker having improved characteristics.
More specifically, the expression xe2x80x9clow-voltage power circuit breakerxe2x80x9d is used to designate a circuit breaker which is generally used in applications, e.g. industrial systems, characterized by operating voltages of less than 1000 volts and by electric currents, typically alternate currents, of relatively high nominal value, (from a few hundred to several thousand amperes), which accordingly produce relatively high power levels.
It is known that power circuit breakers, comprising one or more electric poles, are normally designated to ensure the electric current required by the various users, at the same time performing connection and disconnection of the load or protecting the load from abnormal events, due for example to overloads or to a short circuit, by automatically opening the circuit, or disconnecting the protected circuit by opening appropriately provided electric contacts in order to achieve complete isolation of the load with respect to the power supply grid.
Currently there are many embodiments of low-voltage power circuit breakers, according to the nominal current considered.
In general, however, for each electric pole the interruption of the current, be it a nominal, overload or short-circuit current, occurs by virtue of the separation of a movable contact and of a fixed contact.
An example of a contact arrangement for a low-voltage circuit breaker is described in the European Patent EP 0219449.
The typical structure of an electric pole for a low-voltage power circuit breaker is described with reference to FIG. 1. Said pole comprises a fixed contact 1 and a movable contact 2 which can be mutually coupled/uncoupled. The movable contact 2 is connected and arranged on a contact supporting shaft 4 which is generally made of insulating material and acts as a mechanical support and/or transmits the motion to the movable contact. In order to maintain electrical continuity while allowing the motion of the movable contact 2, a flexible conductor 3 is generally used which has the only task of ensuring the electrical connection between the movable contact 2 and a power supply grid, not shown in FIG. 1.
The movable contact 2, the flexible conductor 3 and the contact supporting shaft 4 are shown in greater detail in FIG. 2. In this case, the flexible conductor 3 is constituted by two flexible metal braids which are accommodated contiguously in a cavity of the contact supporting shaft. At the ends of the metal braids there are a flange 7 and a pivot 8, which are used respectively for connection to the power supply grid and to the movable contact.
In the operating condition for opening at nominal current, following an opening command, the movable contact 2 is moved at a specific speed by the contact supporting shaft 4, which is in turn actuated by an actuation mechanism, not shown in FIGS. 1 and 2.
In the operating condition for opening at short-circuit current, typically before the opening intervention performed by the actuation mechanism, separation of the electric contacts occurs due to the electrodynamic repulsion forces that arise between the movable contact and the fixed contact. For high short-circuit currents, such as those that affect low-voltage power circuit breakers (tens of kiloamperes), these electrodynamic repulsion forces reach values which impart a very high end-stroke velocity to the movable contact.
Due to the very high velocity at the end-stroke, it is necessary to provide, in order to stop the movable contact 2, appropriate arrester devices, for example an abutment plate 5, as shown in FIG. 1, and to give sufficiently sturdy dimensions to the entire kinematic opening system movable contact 2 and contact supporting shaft 4, so that it can withstand the intense stresses caused by the stroke end impact of the movable contact 2. To avoid this occurrence, it is necessary to provide the entire kinematic opening system so that intervention occurs in a relatively short time, particularly before the movable contact reaches the end of its stroke.
These design constrains of course entail a considerable increase in the manufacturing times and costs of the circuit breaker.
Furthermore, again due to the high velocity at the end-stroke, the movable contact might not stop at the abutment plate 5 but might bounce on it, coupling to the fixed contact again. This occurrence would be critical, since it would entail an unwanted closure of the contacts in the presence of fault conditions.
The aim of the present invention is to provide an electric pole for a low-voltage power circuit breaker which allows to limit to relatively modest values the end-stroke velocity of the movable contact during opening in short-circuit operating conditions.
Within the scope of this aim, an object of the present invention is to provide an electric pole for a low-voltage power circuit breaker in which it is possible to ensure the arresting of the movable contact during opening in short-circuit operating conditions without adopting particular arrester devices.
Another object of the present invention is to provide an electric pole for a low-voltage power circuit breaker which ensures the possibility to avoid unwanted reclosures of the electric contacts after opening in short-circuit operating conditions.
Another object of the present invention is to provide an electric pole for a low-voltage power circuit breaker which ensures the possibility to limit the required intervention speed of the kinematic opening system.
Another object of the present invention is to provide an electric pole for a low-voltage power circuit breaker which is highly reliable, simple to manufacture and at low cost.
This aim, these objects and others which will become apparent hereinafter are achieved by an electric pole for a low-voltage power circuit breaker, comprising:
a fixed contact and a movable contact which can be mutually coupled/uncoupled;
at least two conducting elements, suitable to electrically connect said movable contact to a power supply grid, said conducting elements being arranged so that the electric currents flowing through them are equally orientated; and
means for mechanically supporting and actuating said movable contact;
The electric pole according to the invention is characterized in that it comprise at least one insulating element which is interposed between said conducting elements and contrasts, by friction with said conducting elements, electrodynamic repulsion forces that are generated between said fixed contact and said movable contact during opening in short-circuit operating conditions.